Iterative Spectrum Balancing for Digital Subscriber Lines

Dynamic spectrum management (DSM) is an important technique for mitigating crosstalk in DSL. One of the first DSM algorithms proposed, Iterative waterfilling (IW), has a low complexity and demonstrates the spectacular performance gains that are possible. Unfortunately IW tends to be highly suboptimal in mixed CO/RT deployments and upstream VDSL. Another DSM algorithm, Optimal spectrum balancing (OSB), uses a weighted rate-sum to find the theoretically optimal transmit spectra. Unfortunately its complexity scales exponentially with the number of lines in the binder N. Typical binders contain 25-100 lines, for which OSB is intractable. This paper presents a new iterative algorithm for spectrum management in DSL. The algorithm optimizes the weighted rate-sum in an iterative fashion, which leads to a quadratic, rather than exponential, complexity in N. The algorithm is tractable for large N and can be used to optimize entire binders. Simulations show that the algorithm performs very close to the theoretical optimum achieved by OSB

Efficient Equalizers for Single and Multi-Carrier Environments with Known Symbol Padding

The use of a cyclic prefix (CP) to mitigate inter-symbol interference is a technique commonly applied in both single and multi-carrier systems. Recently it has been suggested that the CP be replaced by a pre-defined sequence of known symbols. This technique, referred to as 'Known Symbol Padding' (KSP) inserts a short training sequence (TS) at the beginning of each transmission block for equalizer adaption. This allows for fast tracking of changes in the channel and simple synchronization. In this paper we present purely deterministic equalizers for both single and multi-carrier environments with KSP. We show that, through better utilization of the CP overhead, these equalizers exhibit superior performance to those in a conventional CP system. Low-complexity implementations, particularly for the multi-carrier case are also given

Real-Time Face Recognition Using Eigenfaces

In recent years considerable progress has been made in the area of face recognition. Through the development of techniques like eigenfaces, computer can now compute favourably with humans in many face recognition tasks, particularly those in which large databases of faces must be searched. Whilst these methods perform extremely well under constrained conditions, the problem of face recognition under gross variations in expressions, view and lighting remains largely unsolved. This paper details the design of a real-time face recognition system aimed at operating in less constrained environments. The system is capable of single scale recognition with an accuracy of 94% at 2 frames per second. A description of the system's performance and the issues and problems faced during its development is given

Performance of Crosstalk Cancellation in VDSL

Crosstalk in DSL leads to significant performance degradation and large loses in data-rate. Several crosstalk cancellation techniques have been proposed to address this problem, however, in the existing literature the analysis of these approaches is based on SNR calculations and the SNR-gap approximation. Furthermore, for crosstalk cancellation techniques based on decision feedback, the effect of error propagation is completely ignored. This makes it hard to predict the performance of crosstalk cancellation in real life, and to see if the significant potential gains can actually be realized. To address this problem, this paper uses Monte-Carlo simulation to investigate the performance of the various crosstalk cancellation techniques that have been proposed. The effect of noise-enhancement in zero-forcing crosstalk cancellers and error-propagation in decision-feedback cancellers is examined. The results confirm that a very simple crosstalk cancellation structure can achieve near-optimal performance

Simplified Power Allocation for the DSL Multi-access Channel through Column-wise Diagonal Dominance

In the newest generation of DSL systems, crosstalk is the dominant source of performance degradation. Many crosstalk cancellation schemes have been proposed. These schemes typically employ some form of co-ordination between modems and lead to large performance gains. The use of crosstalk cancellation means that power allocation should be viewed as a multi-user problem. In this paper we investigate optimal (i.e. capacity maximizing) power allocation in DSL systems which employ co-ordination to facilitate crosstalk cancellation. By exploiting certain properties of the DSL channel, it is shown that power allocation can be simplified considerably. The result has each user waterfilling against the background noise only, explicitly ignoring the interference from other users. We show this to be near-optimal for upstream DSL when Central Office (CO) modems are co-ordinated. Compared with conventional waterfilling which is done against the background noise and interference, the performance gains are significant

Improved Linear Crosstalk Precompensation for DSL

Crosstalk is the major source of performance degradation in next generation DSL systems such as VDSL. In downstream communications transmitting modems are co- located at the central office. This allows crosstalk precompensation to be employed. In crosstalk precompensation the transmitted signal is pre-distorted such that the pre-distortion destructively interferes with the crosstalk introduced by the channel. Existing crosstalk precompensation techniques either give poor performance or require modification of customer premises equipment (CPE). This is impractical since there are millions of legacy CPE modems already in use. We present a novel crosstalk precompensation technique based on a diagonalization of the crosstalk channel matrix. This technique does not require modification of CPE. Furthermore, certain properties of the DSL channel ensure that this diagonalizing precompensator achieves near-optimal performance

Autonomous Spectrum Balancing for Digital Subscriber Lines

The main performance bottleneck of modern Digital Subscriber Line (DSL) networks is the crosstalk among different lines (users). By deploying Dynamic Spectrum Management (DSM) techniques and reducing excess crosstalks among users, a network operator can dramatically increase the data rates and service reach of broadband access. However, current DSM algorithms suffer from either substantial suboptimality in typical deployment scenarios or prohibitively high complexity due to centralized computation. This paper develops, analyzes, and simulates a new suite of DSM algorithms for DSL interference channel models called Autonomous Spectrum Balancing (ASB), for both synchronous and asynchronous transmission cases. In the synchronous case, the transmissions over different tones are orthogonal to each other. In the asynchronous case, the transmissions on different tones are coupled together due to intercarrier- interference. In both cases, ASB utilizes the concept of a 'reference line', which mimics a typical victim line in the interference channel. The basic procedure in ASB algorithms is simple: each user optimizes the weighted sum of the achievable rates on its own line and the reference line while assuming the interferences from other users as noise. Users then iterate until the target rate constraints are met. Good choices of reference line parameters are already available in industry standards, and the ASB algorithm makes the intuitions completely rigorous and theoretically sound. ASB is the first set of algorithms that is fully autonomous, has low complexity, and yet achieves near-optimal performance. It effectively solves the nonconvex and coupled optimization problem of DSL spectrum management, and overcomes the bottleneck of all previous DSM algorithms

A near-optimal linear crosstalk precoder for downstream VDSL

This paper presents a linear crosstalk precoder for VDSL that has a low run-time complexity. A lower bound on the data-rate of the precoder is developed and guarantees that the precoder achieves near-optimal performance in 99% of VDSL channels

Optimal Multi-user Spectrum Management for Digital Subscriber Lines

Crosstalk is a major issue in modern DSL systems such as ADSL and VDSL. Static spectrum management, the traditional way of ensuring spectral compatibility, employs spectral masks which can be overly conservative and lead to poor performance. In this paper we present a centralized algorithm for optimal spectrum management (OSM) in DSL. The algorithm uses a dual decomposition to solve the spectrum management problem in an efficient and computationally tractable way. The algorithm shows significant performance gains over existing DSM techniques, e.g. in an upstream VDSL scenario the centralized OSM algorithm can outperform a distributed DSM algorithm such as iterative waterfilling by up to 380%